Key statistical indicators of beach litter survey results

Identification, quantification and analysis of observable anthropogenic debris along swiss river and lakes

The key indicators are common relationships used to provide insight to the most frequent questions using parameters that are taken directly from the survey data. The key indicators are relatively easy to calculate and interpret. In this section we explain the key indicators and use examples from the current data.

Indicators for the most frequent questions

  1. What do you find?
  2. How often do you find it?
  3. How much do you find?
  4. What else do you find?
  5. Where do you find the most?

These are important questions to answer. The survey results are a census of objects that were primarily washed up on the beach. This helps answer the question:

What are we likely to find at the beach?

What are we likely to find in the water?

The key indicators differ between regions and locations. Which may mean that the extent and/or nature of the problem is different from one region to another.

The reliability of these indicators is based on the following assumptions:

  1. The more trash there is on the ground the more a person is likely to find
  2. The survey results represent the minimum amount of trash at that site²
  3. For each survey: finding one item does not effect the chance of finding another³

More information

For information on a specific catchment area or a water feature see the notebook for that catchment area (or make one and send a pull request). For more information on the project visit https://www.plagespropres.ch/ .

¹ The EU guide on monitoring marine litter https://mcc.jrc.ec.europa.eu/documents/201702074014.pdfhttps://mcc.jrc.ec.europa.eu/documents/201702074014.pdf
² There is most likely more trash at the survey site, but certainly not less than what was recorded.
³ Independent observations : https://stats.stackexchange.com/questions/116355/what-does-independent-observations-meanhttps://stats.stackexchange.com/questions/116355/what-does-independent-observations-mean

A brief reminder of the survey method

A survey is a collection of observations. The observations correspond to the objects that were removed and counted during the survey. Each object is placed into one of 260 categories¹. The location, date, survey dimensions and the total number of objects in each category is noted.

Some locations are sampled monthly, other were only sampled once.

The scope and size of the data for this example:

Biel/Bienne is the Aare survey area.

 Results for all surveys between 2020-01-01 and 2021-04-30 from the following catchment areas:

  ['aare']


There were 14,058 objects collecteds from 140 surveys.

Fail rate: likelihood of finding an object

The fail rate is the number of times that an object was found divided by the number of surveys.

What does it mean? The fail rate describes the percent of times that a category was identified in relation to the amount of surveys conducted

Use the fail rate to determine how frequently an object is found within a geographic range

Use the fail rate to indentify objects that are found frequently but in low numbers

Use the fail rate and pcs/m to identify objects that are found infrequently but in important quantities

Consider the summary of the surveys conducted in Biel/Bienne:

There were 3,309 objects removed and identified in the course of 17 surveys from 3 locations.

The objects were placed in to the following 121 of the 261 possible categories:

['G148' 'G23' 'G79' 'G156' 'G27' 'G901' 'G167' 'G31' 'G100' 'G177' 'G35'
 'G922' 'G112' 'G200' 'G50' 'G941' 'G143' 'G22' 'G78' 'G153' 'G25' 'G89'
 'G165' 'G30' 'G175' 'G34' 'G917' 'G106' 'G186' 'G48' 'G940' 'G117' 'G213'
 'G73' 'G152' 'G24' 'G87' 'G159' 'G3' 'G905' 'G170' 'G33' 'G101' 'G178'
 'G38' 'G939' 'G115' 'G211' 'G67' 'G95' 'G4' 'G28' 'G904' 'G37' 'G191'
 'G928' 'G10' 'G908' 'G41' 'G923' 'G21' 'G70' 'G942' 'G32' 'G936' 'G135'
 'G68' 'G98' 'G125' 'G8' 'G103' 'G93' 'G133' 'G66' 'G155' 'G91' 'G916'
 'G208' 'G26' 'G198' 'G142' 'G7' 'G150' 'G59' 'G914' 'G930' 'G197' 'G921'
 'G927' 'G76' 'G12' 'G118' 'G137' 'G161' 'G104' 'G171' 'G53' 'G913' 'G71'
 'G20' 'G49' 'G188' 'G75' 'G43' 'G918' 'G210' 'G2' 'G204' 'G134' 'G96'
 'G116' 'G131' 'G203' 'G919' 'G195' 'G64' 'G929' 'G158' 'G82' 'G81' 'G74']

Calculate the fail rate per object category, for one city, lake and river basin

The fail rate can be calculated for any lake, municipality or river bassin provided you have a sufficient quantity of reliable data. Biel/Bienne is a city on Bielersee in the Aare river bassin. There were multiple surveys from multiple locations within the river bassin.

How much data is sufficient? The data should cover the land use characteristics for the geographic and temporal scale appropriate to the area of study. Local authorities will have insight to land use characteristics that can greatly increase monitoring effiency. How confident we are in the findings is directly related to the quality and the amount of data available.

To calculate the fail rates for Biel, Bielersee and the Aare river basin we just add up the number of times a code was used and divide it by the number of surveys for the city, lake or river basin.

The 10 items identified the most often in Biel/Bienne

Compare the fail-rates of the ten most common items from the 17 surveys in Biel to the fail-rates of those same items for Bielersee the Aare and all other survey areas.

With the exception of fragmented plastics and plastic sheeting the fail rate for the top ten items in Biel/Bienne was greater than the rest of the lake, the river bassin and nationally. This means that, in general, there was a greater chance of finding those objects at Biel/Bienne than most other places.

The fail rate is not the probability of finding one object, it is the most likely estimate (MLE) of the probability of finding one object. The MLE is the best estimate for the probability of a binomial variable (the pass fail rate is a bimomial variable). A complete derivation of the MLE of the binomial variable is beyond the scope of this article but very easy to understand⁴.

A 100% fail rate does not mean that you are guaranteed to find the object, it means that the object was identified in all previous samples.

With that the first two questions are answered:

What do you find on the beach in Biel/Bienne?
See the complete list at the end of this document.


There were 121 categories that were identified at least one time:

['G148' 'G23' 'G79' 'G156' 'G27' 'G901' 'G167' 'G31' 'G100' 'G177' 'G35'
 'G922' 'G112' 'G200' 'G50' 'G941' 'G143' 'G22' 'G78' 'G153' 'G25' 'G89'
 'G165' 'G30' 'G175' 'G34' 'G917' 'G106' 'G186' 'G48' 'G940' 'G117' 'G213'
 'G73' 'G152' 'G24' 'G87' 'G159' 'G3' 'G905' 'G170' 'G33' 'G101' 'G178'
 'G38' 'G939' 'G115' 'G211' 'G67' 'G95' 'G4' 'G28' 'G904' 'G37' 'G191'
 'G928' 'G10' 'G908' 'G41' 'G923' 'G21' 'G70' 'G942' 'G32' 'G936' 'G135'
 'G68' 'G98' 'G125' 'G8' 'G103' 'G93' 'G133' 'G66' 'G155' 'G91' 'G916'
 'G208' 'G26' 'G198' 'G142' 'G7' 'G150' 'G59' 'G914' 'G930' 'G197' 'G921'
 'G927' 'G76' 'G12' 'G118' 'G137' 'G161' 'G104' 'G171' 'G53' 'G913' 'G71'
 'G20' 'G49' 'G188' 'G75' 'G43' 'G918' 'G210' 'G2' 'G204' 'G134' 'G96'
 'G116' 'G131' 'G203' 'G919' 'G195' 'G64' 'G929' 'G158' 'G82' 'G81' 'G74']


How often are those objects found?

The 10 objects that were identified the most frequently:

item
Foil wrappers, aluminum foil                                  1.000000
Cigarette filters                                             1.000000
Food wrappers; candy, snack packaging                         0.941176
Plastic pieces 0.5cm - 2.5cm                                  0.882353
Plastic pieces 2.5cm - 50cm                                   0.882353
Foam polystyrene; expanded beads/balls  2.5cm - 50cm          0.823529
Foamed plastic  for thermal insulation or packing material    0.823529
Foamed polystyrene pieces 0.5cm - 2.5cm                       0.823529
Tobacco; plastic packaging, containers                        0.823529
Industrial sheeting                                           0.823529
Name: Biel/Bienne, dtype: float64

Conclusion: We now know what items were found the most often and that tells us about what we may find at the next survey but we do not know how many of these objects were found at each survey. To do that we need to look at the quantity found as well as size of the survey.

⁴ A very simple explanation of the MLE for a binomial variable: https://www.youtube.com/watch?v=4KKV9yZCoM4

Pieces per meter: How many objects found per length of shoreline

Pieces per meter is the number of objects found at each survey divided by the length of the survey.

What does it mean? Pieces per meter describes the quantity of an object that was found for each meter of shoreline surveyed.

Use pieces per meter to find the objects that were found in the greatest quantities

Use pieces per meter to identify zones of accumulation

Why not use the surface area? The norm internationally is to report the results as quantity of objects per length of shoreline surveyed, usually 100 meters. You can use either one, however if you are looking for comparable data sets your choices may be limited if using the surface area is a requirement. The example here is given in pieces per meter.

Biel/Bienne: the ten objects with the highest median pieces per meter per survey

The pieces per meter (pcs_m) is calculated for each observation at the time the survey is submitted. This value can be taken directly from the survey results. The pcs_m value is not cumulative, therefore we need to use either the mean or the median value. Because we are interested in how many may be found at a single survey we will use the median value for each object at each survey in the city of Biel/Bienne.

The first thing to notice is that not all of the most frequently found objects are found in the greatest quantities.

With this observation we can answer questions 3 and 4: How much do you find? What else do you find?

The average pieces per meter per survey in Biel/Bienne was ~ 6, the minimum value was 0.91 and the max was 9.68

HIGH FAIL RATE & HIGH PIECES PER METER - objects found the most often and in the greatest quantities

1. Plastic pieces 0.5cm - 2.5cm
2. Plastic pieces 2.5cm - 50cm
3. Food wrappers; candy, snack packaging
4. Cigarette filters
5. Industrial sheeting
6. Foamed polystyrene pieces 0.5cm - 2.5cm

Combined they had an average pieces per meter per survey of 2.64, a min of 0.34 and max of 5.72

LOW FAIL RATE & HIGH PIECES PER METER - objects found less often but in important quantities

1. Construction material; bricks, pipes, cement
2. Granules <5mm
3. Paraffin wax
4. Packaging plastic nonfood or unknown

Combined they had an average pieces per meter per survey of 0.36, a min of 0.02 and max of 1.09

HIGH FAIL RATE & LOW PIECES PER METER - objects found often but in reduced quantities:

1. Foamed plastic  for thermal insulation or packing material
2. Tobacco; plastic packaging, containers
3. Foil wrappers, aluminum foil
4. Foam polystyrene; expanded beads/balls  2.5cm - 50cm

Combined they had an average pieces per meter per survey of 0.37, a min of 0.02 and max of 0.9

By combining the average pieces per meter per survey of the results from above we are accounting for on average ~3.4 out of the 6 pieces per meter.

What about the rest? There were 121 different categories used, so far we have accounted for 2,279 of the 3,309 objects (68%) with only 14 categories. That leaves ~ 1,030 objects spread out between 107 categories⁵.

This result is not uncommon, a relatively small group of objects make up a large percentage of the objects found. This is true even in the marine environment a quick check of the survey results from Marine Litter Watch⁶ reveals that the top ten in Biel/Bienne share four common objects with the top ten items in the EU:

  1. Food wrappers; candy, snack packaging
  2. Foam polystyrene; expanded beads/balls 2.5cm - 50cm
  3. Cigarette butts and filters
  4. Plastic pieces 2.5cm - 50cm

⁵ Find the full list of objects found at the end of this article
https://www.eea.europa.eu/themes/water/europes-seas-and-coasts/assessments/marine-litterwatch/data-and-results/marine-litterwatch-data-viewer

Putting it all together

The pieces per meter and fail rate help define the trash removed with more precision. The fail rate for an item or a group of items is an indicator of how likely we are to find the item or group of items. Pieces per meter gives a reference value for the minimum number of objects we expect to find given the previous results. The more samples we have the more confident we can be in our assumptions.

This opens the possibility of measuring mitigation techniques and reduction strategies using both of these indicators. The pieces per meter and the fail rate can vary from one location to another and from one season to another. If both the fail rate and the pieces per meter decline for a group of objects or an object year over year, it is safe to assume that the abundance of the object in the environment is in decline.

Next steps

Understanding how the fail rate and pcs/m vary in response to things like population, infrastructure or time of the year.

Grouping codes together to represent economic sectors or usage.

Practical excercise 1: Where would you go to find plastic industrial pellets on Bielersee?

Plastic industrial pellets are the primary material used to produce plastic objects. They are disc or pellet shaped with a diameter of ~5mm.

Given the following survey results and the map of survey locations for Bielersee and assuming the surveys were done according to protocol:

  1. Where do you think your chances are the best of finding one?
  2. Suppose you did a survey of 50 meters. How many would you expect to find?
  3. Why did you pick that location? How sure are you of your choice?
# of samples # fail median pcs/m #found
location
bielersee_vinelz_fankhausers 12 9 0.127500 22
camp-des-peches 1 0 0.000000 0
erlach-camping-strand 1 0 0.000000 0
gals-reserve 2 0 0.000000 0
ligerz-strand 2 0 0.000000 0
luscherz-plage 4 2 0.015000 3
luscherz-two 1 0 0.000000 0
mullermatte 13 9 0.066923 37
nidau-strand 1 1 0.080000 2
strandboden-biel 2 2 0.065000 7



Have a great day

This project was made possible by the Swiss federal office for the environment.

This document originates from https://github.com/hammerdirt-analyst/iqals all copyrights apply.

Produced by: roger@hammerdirt.ch
Date: 2021-05-27

Statistics is fun when you do it outside



Biel/Bienne: all objects identified

item quantity pcs_m fail fail_rate
code
G27 Cigarette filters 781 1.150 17 1.000000
G30 Food wrappers; candy, snack packaging 253 0.415 16 0.941176
G79 Plastic pieces 2.5cm - 50cm 205 0.320 15 0.882353
G78 Plastic pieces 0.5cm - 2.5cm 185 0.220 15 0.882353
G67 Industrial sheeting 166 0.255 14 0.823529
G81 Foamed polystyrene pieces 0.5cm - 2.5cm 110 0.165 14 0.823529
G941 Packaging plastic nonfood or unknown 95 0.140 11 0.647059
G200 Glass or ceramic drink bottles, pieces 94 0.080 13 0.764706
G940 Foamed EVA (flexible plastic) for crafts & wat... 84 0.090 7 0.411765
G213 Paraffin wax 75 0.145 10 0.588235
G117 Styrofoam < 5mm 71 0.100 11 0.647059
G25 Tobacco; plastic packaging, containers 62 0.070 14 0.823529
G74 Foamed plastic for thermal insulation or pack... 62 0.075 14 0.823529
G95 Cotton bud/swab sticks 55 0.100 13 0.764706
G177 Foil wrappers, aluminum foil 54 0.060 17 1.000000
G82 Foam polystyrene; expanded beads/balls 2.5cm ... 53 0.060 14 0.823529
G89 Plastic construction waste 49 0.090 13 0.764706
G156 Paper fragments 47 0.090 10 0.588235
G112 Industrial pellets (nurdles) 44 0.060 11 0.647059
G106 Plastic fragments angular <5mm 38 0.070 11 0.647059
G73 Foamed items & pieces (non packaging/insulatio... 37 0.070 10 0.588235
G31 Lollypop sticks 33 0.050 13 0.764706
G24 Lid/cap rings from plastic bottles/containers 28 0.050 11 0.647059
G35 Straws and stirrers 27 0.050 11 0.647059
G904 Fireworks; rocket caps, exploded parts & packa... 24 0.025 10 0.588235
G50 String < 1cm 22 0.030 11 0.647059
G23 Lids unidentified 22 0.020 9 0.529412
G178 Metal bottle caps, lids & pull tabs from cans 21 0.030 12 0.705882
G204 Construction material; bricks, pipes, cement 20 0.315 2 0.117647
G125 Balloons and balloon sticks 20 0.040 8 0.470588
G10 Food containers single use foamed or plastic 19 0.030 11 0.647059
G33 Cups, lids, single use foamed and hard plastic 16 0.050 8 0.470588
G22 Lids for chemicals, detergents (non-food) 16 0.020 9 0.529412
G211 Other medical (swabs, bandaging, adhesive plas... 16 0.030 11 0.647059
G159 Corks 15 0.035 8 0.470588
G152 Cigarette boxes, tobacco related paper/cardboard 14 0.050 4 0.235294
G191 Wire and mesh 14 0.045 8 0.470588
G21 Drink lids 14 0.040 8 0.470588
G66 Straps/bands; hard, plastic package fastener 13 0.030 8 0.470588
G87 Tape, masking/duct/packing 13 0.030 7 0.411765
G32 Toys and party favors 12 0.030 8 0.470588
G153 Cups, food containers, wrappers (paper) 12 0.030 5 0.294118
G923 Tissue, toilet paper, napkins, paper towels 11 0.030 5 0.294118
G3 Bags; plastic shopping/carrier/grocery and pieces 10 0.030 5 0.294118
G922 Labels, bar codes 10 0.020 9 0.529412
G161 Processed timber 10 0.050 4 0.235294
G98 Diapers - wipes 9 0.030 7 0.411765
G75 Plastic/polystyrene pcs 0 - 2.5 cm 8 0.100 2 0.117647
G70 Shotgun cartridges 8 0.035 6 0.352941
G905 Hair clip, hair ties, personal accessories pl... 8 0.020 6 0.352941
G942 Plastic shavings from lathes, CNC machining 8 0.050 5 0.294118
G186 Industrial scrap 8 0.030 5 0.294118
G148 Cardboard (boxes and fragments) 7 0.020 3 0.176471
G165 Ice cream sticks, toothpicks, chopsticks 7 0.020 7 0.411765
G4 Small plastic bags; freezer, zip-lock etc. 7 0.050 4 0.235294
G175 Cans, beverage 7 0.040 4 0.235294
G118 Small industrial spheres <5mm 7 0.050 3 0.176471
G7 Drink bottles < = 0.5L 6 0.025 4 0.235294
G908 Tape; electrical, insulating 6 0.025 4 0.235294
G101 Dog feces bag 6 0.030 3 0.176471
G170 Wood (processed) 6 0.030 3 0.176471
G100 Medical; containers/tubes/ packaging 6 0.020 5 0.294118
G198 Other metal pieces < 50cm 6 0.030 4 0.235294
G26 Cigarette lighters 6 0.030 5 0.294118
G28 Pens, lids, mechanical pencils etc. 6 0.025 6 0.352941
G76 Plastic/foamed polystyrene 2.5 > < 50 5 0.020 3 0.176471
G928 Ribbons and bows 5 0.025 4 0.235294
G91 Biomass holder 5 0.030 5 0.294118
G115 Foamed plastic <5mm 5 0.025 4 0.235294
G901 Mask medical, synthetic 5 0.020 4 0.235294
G93 Cable ties; steggel, zip, zap straps 4 0.020 4 0.235294
G2 Bags 4 0.060 2 0.117647
G116 Granules <5mm 4 0.160 1 0.058824
G203 Tableware ceramic or glass, cups, plates, pieces 4 0.090 1 0.058824
G142 Rope , string or nets 4 0.070 2 0.117647
G64 Fenders 3 0.090 1 0.058824
G917 Terracotta balls 3 0.035 2 0.117647
G143 Sails and canvas 3 0.020 3 0.176471
G34 Cutlery, plates and trays 3 0.020 3 0.176471
G20 Caps and lids 3 0.030 3 0.176471
G49 Rope > 1cm 3 0.070 1 0.058824
G131 Rubber bands 3 0.120 1 0.058824
G133 Condoms incl. packaging 3 0.020 3 0.176471
G919 Nails, screws, bolts etc. 3 0.060 1 0.058824
G68 Fiberglass fragments 3 0.030 3 0.176471
G167 Matches or fireworks 3 0.070 1 0.058824
G930 Foam earplugs 2 0.020 2 0.117647
G914 Paperclips, clothespins, plastic utility items 2 0.025 2 0.117647
G135 Clothes, footware, headware, gloves 2 0.030 2 0.117647
G210 Other glass/ceramic 2 0.025 2 0.117647
G96 Sanitary pads /panty liners/tampons and applic... 2 0.035 2 0.117647
G12 Cosmetics, non-beach use personal care containers 2 0.020 2 0.117647
G103 Plastic fragments rounded <5mm 2 0.030 2 0.117647
G48 Rope, synthetic 2 0.020 2 0.117647
G936 Sheeting ag. greenhouse film 2 0.025 2 0.117647
G188 Other cans < 4 L 2 0.050 1 0.058824
G59 Fishing line monofilament (angling) 2 0.025 2 0.117647
G927 String trimmer line, used to cut grass, weeds,... 2 0.050 1 0.058824
G197 Other metal 1 0.020 1 0.058824
G195 Batteries - household 1 0.020 1 0.058824
G939 Flowers, plants plastic 1 0.020 1 0.058824
G929 Electronics and pieces; sensors, headsets etc. 1 0.030 1 0.058824
G921 Ceramic tile and pieces 1 0.020 1 0.058824
G104 Plastic fragments subrounded <5mm 1 0.020 1 0.058824
G150 Milk cartons, tetrapack 1 0.020 1 0.058824
G53 Nets and pieces < 50cm 1 0.020 1 0.058824
G918 Safety pins, paper clips, small metal utility ... 1 0.030 1 0.058824
G916 Pencils and pieces 1 0.020 1 0.058824
G913 Pacifier 1 0.020 1 0.058824
G38 Coverings; plastic packaging, sheeting for pro... 1 0.020 1 0.058824
G41 Glove industrial/professional 1 0.020 1 0.058824
G43 Tags fishing or industry (security tags, seals) 1 0.020 1 0.058824
G8 Drink bottles > 0.5L 1 0.020 1 0.058824
G208 Glass or ceramic fragments > 2.5 cm 1 0.020 1 0.058824
G134 Other rubber 1 0.040 1 0.058824
G71 Shoes sandals 1 0.020 1 0.058824
G158 Other paper items 1 0.030 1 0.058824
G137 Clothing, towels & rags 1 0.020 1 0.058824
G155 Fireworks paper tubes and fragments 1 0.020 1 0.058824
G171 Other wood < 50cm 1 0.020 1 0.058824
G37 Mesh bags 1 0.020 1 0.058824